1. Field of the Invention
The present invention relates to a power amplifier and a method for realizing the same and, more particularly, to a method of using a generalized multi-port feedback circuit device to make a power amplifier unilateral.
2. Description of Related Art
Feedback techniques were used to implement the unilateral amplifiers. There are basically three types of feedback amplifiers. The first is the high gain amplifier (HGA), which was presented by Mason in 1954. Feedback and matching circuits are used to make the input and output reflection loss (S11 and S22) and the reverse gain (S12) zero, and unilateral gain is simultaneously accomplished. Therefore, the amplifier becomes an ideal and unilateral amplifier.
The second is the low noise amplifier (LNA), which was presented by Vendelin in 1975, gives the lowest noise figure (NF) and S11′=0. In the design idea of conventional low noise amplifiers, because the optimum matching points of noise and gain are not identical, a compromise should always be made between noise and gain when designing a low noise amplifier. Therefore, the feedback circuit idea of linear amplifiers is also applied in the design of low noise amplifiers afterwards. Through the help of a feedback circuit, the optimum matching points of noise and gain can be made identical so as to simultaneously accomplish matching of noise and gain. In 2004, a paper of M. P. Van der Heijden also achieved unilateral function of low noise amplifier.
In 1986, Texas Instruments Incorporated disclosed a patent of common source feedback circuit and applied it to low noise amplifiers. This is the first example of using a single chip to manufacture a feedback low noise amplifier. A similar patent was disclosed by General Electric Corporation in 1997, in which a similar single chip low noise amplifier was proposed. Why the linear feedback circuit can be used in low noise amplifiers is that the power of noise itself is very small so that the noise source can be approximated as a linear voltage or current source. Therefore, the linear circuit theory can be applied in low noise amplifiers.
The third is the high power amplifier (HPA). In the design of a microwave emitter, the power amplifier determines the output power of the emitter. However, a compromise should always be made between output power, power gain and impedance matching when using a power amplifier of the conventional architecture. Until recently, feedback and unilateralization of power amplifier has never been fully treated. Because power amplifiers will encounter the problem of nonlinearity, the linear circuit theory cannot be used in the design of power amplifier.
FIG. 1 is a diagram of a conventional power amplifier. As shown in FIG. 1, through the measurement of load pull of the power transistor, the load admittance (Yopt) capable of achieving the maximum output power can be calculated. The load admittance capable of achieving the maximum output power, however, can not accomplish optimization of gain and impedance matching.
As can be known from the above examples, feedback techniques are indispensable to the design and improvement of amplifiers. Accordingly, the present invention aims to provide a feedback method and device capable of realizing unilateralization of power amplifier.